This invention comprehends the foaming of unsaturated polyesters using acid sensitive azoxy compounds. It also relates to a mono- or di-, unsymmetrical azoxy compound.
Prior to the present invention, polyester resins were foamed using complex mixtures of components in aqueous systems. Many of the prior art references relate to a sequential operation in which gas is first released in the polymerizable cross-linkable resinous medium followed by an attempt to effect the polymerization/crosslinking without destroying the foam. The foaming operation and the polymerization/crosslinking operation had to be correlated in order to obtain any successful results; such operations are difficult and have been found to be entirely impractical. Another prior art attempt to producing foam is by simultaneously initiating the polymerization/crosslinking and releasing of the gas into the resinous medium. This, too, has proved to be entirely impractical because the polymerization/crosslinking reaction takes place so quickly that the resinous medium becomes too highly viscous or even rigid at such an early stage that the resin cannot be foamed. In the case where the operations were performed sequentially, the foam was a transitory nature, i.e., foam would dissapate, to the extent that the gas would be released to the atmosphere, before polymerization/crosslinking could be effected.
U.S. Pat. No. 3,993,609 describes a process similar to the process of this invention for foaming a polymerizable medium by employing acid sensitive azo compounds in acidulous media. A considerable number of these azo compounds, however, are thermally unstable and require refrigeration. In contrast thereto, the corresponding azoxy compounds are much more thermally stable and can be stored and shipped at room temperature or above which make them economically more feasible.